In planar optical waveguides, Bragg gratings are used, for example, to filter out of chromatic light a fraction with a defined wavelength. This wavelength is referred to as the Bragg wavelength and is determined by the period of the Bragg grating. In optical frequency-division multiplexing communication systems, such wavelength filters can be used as integrated optical multiplexers or demultiplexers.
There are various ways of producing Bragg gratings in planar optical waveguides. One possibility is to provide the core of the optical waveguide with periodically disposed recesses as is described, for example, in EP-B1-0 546 705. From EP-A1-0 701 150 it is known to apply a high-index coating to the vertical surfaces of these recesses.
Another approach uses the photosensitivity of the waveguide core. A material is called photosensitive if its refractive index can be increased by irradiation with light. The photosensitivity will generally be highest if the material is irradiated with short-wavelength light as is generated by UV lasers, for example. The core layers of most conventional optical waveguide structures are photosensitive; however, the photosensitivity can also be increased selectively by taking suitable measures, as will be explained below.
To imprint a Bragg grating in the photosensitive core of an optical waveguide with the aid of UV light, an optical interference pattern is formed in the core. This can be done, for example, by coupling laser light into the core. If the light wave is reflected at the end of the waveguide opposite the coupling point, a standing wave in which intensity maxima and minima follow each other at regular intervals will be produced in the waveguide. Such a method is described for optical fibers in U.S. Pat. No. 4,474,427.
Another method of producing an optical interference pattern consists of directing collimated UV light through a phase mask to form an interference pattern at the rear of the phase mask, to which the waveguide structure is exposed. At those points where the light waves are constructively superposed in the interference pattern, the light intensity is so high that the refractive index in the waveguide core changes permanently.
Such a fabrication process using a phase mask is described in DE-A1-43 37 103. FIG. 7 shows a waveguide structure deposited on a substrate SUB and consisting of a lower buffer layer BUF, a core COR, and an upper buffer layer (=cladding layer) CLA. Disposed directly above the waveguide structure is a phase mask PM in the form of a quartz body provided with a surface-relief structure. The surface-relief pattern approximately corresponds to the pattern of a periodic rectangular wave with a period of about 1000 nm. This phase mask PM is irradiated with UV light from an excimer laser. Below the phase mask PM, i.e., within the waveguide structure, an interference pattern is formed. If the individual parameters of the process (including period and amplitude of the surface-relief structure, wavelength of the laser) are chosen appropriately, the diffracted zerothorder beams indicated in FIG. 7 as FOR can be largely suppressed, so that nearly exclusively the divergent first-order beams SOR will impinge on the core COR of the optical waveguide and imprint a Bragg grating GRA therein.
With the process described in DE-A1-43 37 103, highly precise Bragg gratings can be produced in optical waveguides in a relative simply manner. A disadvantage is, however, that such a phase mask permits the fabrication of only one Bragg grating with a given Bragg wavelength at a time. For certain applications, however, it is necessary to imprint several Bragg gratings with different Bragg wavelengths in a confined space. An example of such an application is a cascaded add&drop multiplexer, which will be dealt with in more detail below. If several Bragg gratings with different Bragg wavelengths are to be imprinted in a confined space using the method just described, a different phase mask must be used for each Bragg grating. In addition, it is very difficult in that case to precisely position the individual phase masks relative to the core of the optical waveguide.
A remedy for this is provided by a method disclosed in EP-A2-736 783. There, a mask is first applied to the clad layer of a planar optical waveguide using conventional photolithographic techniques. The planar optical waveguide is then irradiated through the mask with light of a suitable wavelength. The effect is the same as with the use of a phase mask, i.e., by forming an interference pattern,.a Bragg grating is imprinted in the photosensitive core of the optical waveguide. Thus, the phase mask is functionally integrated with the optical waveguide and, therefore, need not be aligned. A disadvantage is that the mask applied to the optical waveguide is not resistant to aging. Therefore, the waveguide must be irradiated with light immediately after fabrication of the mask. The possibility to determine the Bragg wavelength of the grating much later via the wavelength of the light used for irradiation does not exist. In addition, a considerable part of the light is reflected or absorbed by the mask. The efficiency with which the Bragg grating is imprinted is correspondingly low, and the refractive-index differences obtainable are correspondingly small. An arbitrary increase in light power is not possible since this could damage the mask.